1. Field of the Invention
The present invention relates to a signal detecting apparatus, and is applicable to an intermediate frequency detecting circuit for detecting, for example, a video intermediate frequency signal containing a video and an audio used in a color television receiver and so on.
2. Description of the Relative Art
A conventional color television receiver receives a vestigial side-band (VSB) broadcasting signal by a tuner and demodulates a video intermediate frequency signal (VIF signal) containing a sound carrier (f.sub.A) and a chrominance subcarrier (f.sub.SC). Thus, beats of, for example, 920 [KHz] is generated in proportional to the magnitude of the sound carrier and the chrominance subcarrier. When the beats change the envelope of carrier chrominance signal, non-linear distortion is generated in a luminance signal and a chrominance signal, which appears as stripes due to beat interference on the screen.
To reduce the beat interference, it is sufficient to make the sound carrier small in contrast to the picture carrier in the VIF signal, and then the signal is input in the video intermediate frequency detecting circuit for detection. Therefore, the color television receiver supplies the VIF signal to a surface acoustic wave filter (hereinafter referred to as a SAW filter) for waveform shaping so that the sound carrier and the chrominance subcarrier are significantly attenuated as compared with that in the monochrome television.
In NTSC system and PAL system, upper and lower side bands of the carrier chrominance signal need to be symmetry when demodulating the chrominance signal to eliminate crosstalk between two chrominance signals which are obtained by quadrature two-phase modulation of the chrominance subcarrier. Thus, the SAW filter having characteristics in which the amplitude of chrominance subcarrier frequency is at -3 [dB] to -6 [dB] from the level is selected, so that the setting range for setting the magnitude of sound carrier and chrominance subcarrier is limited.
Recently, the television is increased in its size so that high picture quality is required even for ground wave broadcasting. Particularly in the tuner systems, the video characteristics depend on the video intermediate frequency detecting circuit. There is a video intermediate frequency detecting circuit with an inter-carrier system simply arranged by using a PLL circuit.
This type of video intermediate frequency detecting circuit provides a VIF signal, for example, to two multipliers and the PLL circuit respectively, and provides the output from the PLL circuit to two multipliers to detect an image. This results in an ability to obtain the video signal from one of the multipliers and an audio signal by demodulating a signal of difference frequency between above signal and the signal taken out from the other adder.
However, since the margin of setting range is narrow when the attenuation level of the sound carrier and the chrominance subcarrier is set in the SAW filter as described above, the sound carrier and the chrominance subcarrier remain in the VIF signal which has passed through the SAW filter. In addition, since it is necessary to extract the sound signal while assuring sufficient S/N ratio, the sound carrier is never sufficiently attenuated. Thus, a voltage control oscillator (hereinafter referred to as a "VCO") which constitutes the PLL circuit in the aforementioned video intermediate frequency detecting circuit operates based on the VIF signal in which the sound carrier and the chrominance subcarrier remain, and outputs a switching carrier containing the sound carrier and the chrominance subcarrier to the multiplier.
Accordingly, there occurs the problem that the video signal obtained by detecting with the multiplier to which the switching carrier is given contains a beat component having a frequency which corresponds to the difference between the sound carrier and the chrominance subcarrier and the image quality is thus deteriorated.
To avoid this type of problem, it is possible to reduce the components of the sound carrier and the chrominance subcarrier by increasing carrier purity of the VCO. However, such method narrows the operating range of the PLL circuit, and causes a disadvantage that it is difficult to compatibly set both the stable operation of the VCO and reduction of the components of sound carrier and the chrominance subcarrier.
Instead of the inter-carrier system, it is also possible to respectively use the SAW filters exclusively used for picture and sound, and to sufficiently reduce the sound carrier in the VIF signal with the picture SAW filter. With this method, it is possible to reduce the beat interference in a picture while maintaining S/N ratio of the audio output signal.
However, this method has a disadvantage that requirement of the exclusive SAW filters makes the structure complicated. Also, in this method, since wiring pattern for high frequency signals becomes complicated in mounting on a board, there arise problems that signals are easily coupled, and that a board is increased in its area when the circuit is implemented in an integrated circuit.
Incidentally, the sound carrier in the VIF signal between television broadcasting system M is 4.5 [MHz], and the sound carrier in the VIF signal between television broadcasting system B and system G are 5.5 [MHz], respectively. In addition, the chrominance subcarrier of the NTSC system is 3.58 [MHz] and the chrominance subcarrier of the PAL system is 4.43 [MHz], respectively.